Dirty Tricks in the Plankton: Diversity and Role of Marine Parasitic Protists

Parasitism is an immensely successful mode of nutrition and parasitic organisms are abundant in most ecosystems. This is also the case for marine planktonic ecosystems in which a large variety of parasitic species are known. Most of these parasites are protists and they infect a wide range of hosts from the marine plankton, ranging from other protists to larger planktonic invertebrates. Parasites often have morphologies and life cycles that are highly specialized as compared to their free-living relatives. However, this does not mean that parasites are necessarily odd or rare phenomena; on the contrary parasites constitute numerically and ecologically important components of the ecosystem. This review gives an overview of the existing knowledge on the diversity and occurrence of parasitic protists in the marine plankton and examines the available information on the potential effects and role of parasitism in this ecosystem. Importance is given to the fact that prevalence and impact of parasitic organisms in marine planktonic systems appear to be overwhelmingly understudied.